Unsplash photos are public domain:

Jonas Nilsson Lee <https://unsplash.imgix.net/reserve/QGXfT1CkRpmvlwtPpgul_IMG_0558.jpg?q=75&fm=jpg&s=25c25f99c3faba09b92aacf40a9e9de5>
Mr. Lee <https://unsplash.imgix.net/reserve/u8GBf8bBSGy3YCIhyZeI_IMG_8737.jpg?q=75&fm=jpg&s=7b4763d2519f224c237df3f360f65ad0>
Pablo Garcia Saldaña <https://unsplash.com/photos/15d_4S2tJsQ>
Rob Bye <https://unsplash.com/photos/Kc7xqFTtcc4>
Ryan Schroeder <https://unsplash.com/photos/Gg7uKdHFb_c>
Samuel Zeller <https://unsplash.imgix.net/photo-1418225085675-d9ee79523efb?q=75&fm=jpg&s=aa95d693e0f73036db1efad137c4a765>
Photo by SpaceX <https://unsplash.com/photos/VBNb52J8Trk>

With permission from "bo0xvn":

94 <http://bo0xvn.deviantart.com/art/94-206184488>
164 <http://bo0xvn.deviantart.com/art/164-264533610>
168 <http://bo0xvn.deviantart.com/art/168-265068449>
173 <http://bo0xvn.deviantart.com/art/173-267532246>

Deviantart:

The Coast <http://skyofca.deviantart.com/art/The-Coast-197763965>
Leaves <http://emats.deviantart.com/art/Leaves-Wallpaper-100824718>
Urban Dream <http://delta909.deviantart.com/art/UrbanDream-190307415>
Water Lilly <http://islingt0ner.deviantart.com/art/Water-Lily-165791833>

Flickr:

Horseshoe Bend Sunset <http://www.flickr.com/photos/24046097@N00/4418745305>
Lake Tahoe Colors <https://www.flickr.com/photos/romainguy/4247339382/>
Living Stones <https://www.flickr.com/photos/andesine/5560436312>

Other/Unknown Origin:
Tunnel
Wild Night <https://dl.dropboxusercontent.com/u/11000978/Wild%20Night%20%282880x1800%29.jpg>
